Synchronous shielding in vacuum deposition system

ABSTRACT

A sputtering apparatus of the bell-jar type includes a high-vacuum pump disposed centrally of, and directly within a vacuum chamber of the apparatus. A separate enclosing member is disposed within the chamber and allows selective exposure of the pump to the chamber. An annular workholder is mounted in concentric surrounding relation with the pump, and an apertured, annular shutter is mounted in concentric surrounding relation with the workholder. Targets of materials to be sputter deposited on the workpieces are mounted on the inside wall of the main enclosure of the apparatus. The shutter and the workholder are moved in synchronism at the beginning and at the end of the deposition cycle to provide a uniform exposure of all the workpieces to the targets.

BACKGROUND OF THE INVENTION

This invention relates to a vacuum treating system, and particularly,but not limited to a, high production, high quality sputtering methodfor depositing materials on substrates.

Various vacuum treating apparatus, including various conventionalsputtering apparatus, require a vacuum pump down each time a batch ofworkpieces is loaded into the apparatus. The pump down time can be quiteextensive, often being an appreciable portion of the entire processingsequence, and it is thus desirable to reduce it. This is one object ofthis invention.

Obtaining reproducible results from workpiece to workpiece requires thatall the workpieces be treated or operated on in substantially identicalfashion. One problem heretofore existing in certain types of vacuumtreating apparatus in which different workpieces of a batch ofworkpieces are successively operated on over a period of time is that ofproviding uniform treating conditions throughout the treating period,particularly at the beginning and end of the period.

A further object of this invention is to provide uniform andreproducible treatment of different workpieces successively operated onin a vacuum treating process.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a sectional elevational view of a sputtering apparatus inaccordance with one embodiment of this invention;

FIG. 2 is a sectional view taken along line 2--2 of FIG. 1;

FIG. 3 is a front view of a workpiece holder, or pallet, used in theapparatus shown in FIGS. 1 and 2;

FIG. 4 is an enlarged view of a back portion of the pallet shown in FIG.3;

FIG. 5 is a schematic plan view of a portion of the apparatus showingthe relative positioning of rotatable portions thereof at the start ofan operating cycle; and

FIGS. 6 and 7 are views similar to that of FIG. 5 showing the apparatusat successive portions of the operating cycle.

PREFERRED EMBODIMENTS OF THE INVENTION

Shown in FIG. 1 is a sputtering apparatus 10 of the present invention.The apparatus 10 is basically of the bell jar type, and comprises acup-shaped, generally cylindrical main enclosure 12 in removable,hermetically sealable relation with a base plate 14. A pneumatic liftingmeans 15 of known type is provided for lifting the enclosure 12 awayfrom the plate 14 and exposing the work treatment chamber of theapparatus.

To pump down the apparatus 10, conventional pump means can be used. Forexample, to pump down the apparatus from atmospheric pressure to an"intermediate" level of pressure, e.g., about 100 torr, a conventionalroughing pump (not shown) can be used coupled to the apparatus via aduct 16 through the base plate 14. To pump down the apparatus to afurther reduced pressure, e.g., about 5 × 10⁻ ⁵ torr, a knowncombination of a turbomolecular pump backed up by a foreline pump(neither pump being shown) can be used coupled to the apparatus via duct17 (see FIG. 2). To achieve a relatively "high" vacuum, e.g., about 5 ×10⁻ ⁷ torr to "clean out" the apparatus prior to the actual operationthereof, a high vacuum pump 18 (FIG. 1) is also provided. In accordancewith this invention, the high vacuum pump 18, e.g., a cryogenic pump ofknown type described in some detail hereinafter, is mounted on the baseplate 14 interiorly of the apparatus itself. The importance of thisdisposition of the pump within the apparatus is described hereinafter.

To isolate the pump 18 from the ambient air when the enclosure 12 islifted, as well as to isolate the pump from surrounding portions of theapparatus 10 during its operation, as described below, a separate capmember or enclosure 20 is provided which forms an hermetic seal with thebase plate 14. To substantially fully expose the pump 18 to thesurrounding portions of the apparatus, the enclosure 20 is lifted (asshown in phantom in FIG. 1) within the main enclosure 12 by means of adrive screw 26 driven by a motor 28 mounted beneath the base plate 14. Abellows 30 hermetically sealed to the screw 26 within the apparatuspreserves the hermeticity thereof as the screw is advanced or retractedwithin the apparatus.

Four cathode target assemblies 34, 36, 38 and 40 (FIG. 2), eachincluding a target 34a, 36a, 38a and 40a, respectively, of a material tobe sputter-deposited onto workpieces within the apparatus 10, aremounted inside the apparatus 10 on the side wall 42 of the mainenclosure 12. The target assemblies comprise the "work treating means"in this embodiment of the invention. Electrical connections and watercooling means are coupled to the target assemblies through the wall 42.In this embodiment of the invention the target assemblies are angularlydisposed 90° from each other, the two oppositely disposed assemblies 34and 38 including targets 34a and 38a of titanium, and the two oppositelydisposed assemblies 36 and 40 including targets 36a and 40a of platinum.Each of the targets extends generally parallel to the longitudinal axisof the apparatus. More than four target assemblies, and targets ofdifferent materials, can be used, as desired.

The target assemblies are designed in accordance with known principlesand can comprise, e.g., a hollow housing through which cooling water iscirculated. The targets comprise, e.g., a rectangular plate of thematerial to be sputter deposited secured to the interior facing wall ofthe target assembly housing. Illustrative dimensions of the varioustargets are provided hereinafter.

While any number of different workpieces, or a single workpiece, can beoperated on by the instant apparatus, in this embodiment of theinvention the workpieces comprise 3 inch diameter disc-like wafers 44(FIG. 4) of semiconductor material. Workholders for the wafers 44include a known type of pallet 46 (FIGS. 3 and 4), each pallet being arelatively thin, rectangular plate containing a number of counterboredapertures 50 therethrough, the smaller diameter portion 51 of theapertures 50 on the "front" side (FIG. 3) of the pallet 46 providing aledge 51' for receipt of the wafers. Each wafer 44 is held in place(FIG. 4) against the ledge 51' by means of a resilient wire 52 extendingacross the aperture 50 between dove-tail shaped grooves 53 on the backside of the pallet.

The loaded pallets 46 are mounted, in turn, on a rotatable annularplatform or "carrousel" 54 (FIG. 1) within the apparatus 10, thedifferent pallets being disposed in a side-by-side, vertical,cylindrical "work assembly" or array on the carrousel. Simple springdetent means (not shown) can be used to hold the pallets in place. For 3inch wafers 44, each pallet 46 measures 35/8 by 143/4 inches, thecarrousel 54 having an outer diameter of about 30 inches, and mountingthereon 26 pallets 46.

The annular carrousel 54 is mounted concentrically around the pump 18 onbearings 56 mounted, in turn, on a number of support posts 58 spacedaround the pump 18. The carrousel 54 is rotatable by means of a gear 60engaged with a gear track 62 on the inside surface of the annularcarrousel, the gear 60 being driven by a motor 64 mounted beneath thebase plate 14. As described in greater detail below, the carrousel 54rotates the various pallets 46 successively past different ones of thetarget assemblies 34, 36, 38 and 40, whereby materials sputtered fromthe targets are deposited onto the wafers carried by the pallets.

Cooperating with the carrousel 54 for controlling the uniformity andpurity of material deposited onto the wafers 44 at the beginning and endof the processing cycles, as described hereinafter, is a cylindricalshutter 66 mounted on an annular, rotatable platform 68. The platform 68is mounted on bearings 70 mounted on support posts 72 spaced around thepump 18, the shutter 66 being disposed, as shown, in concentricsurrounding relation with the array of pallets 46 on the carrousel 54.The inside surface of the annular shutter platform 68 is provided with agear track 74, and rotation of the platform 68 and the shutter 66thereon is by means of a gear 76 driven by a motor 78 mounted beneaththe base plate 14. The shutter 66 has an outer diameter of 33 inches, awall thickness of 1/16 inch, and is spaced from each target (the centersof the flat plates thereof) a distance of 11/4 inches.

As shown in FIG. 2, the cylindrical shutter 66 has two rectangularapertures 80 and 82 therethrough disposed 180° apart, each apertureextending in the longitudinal direction and corresponding generally inshape and dimensions with the targets of the target assemblies 34, 36,38 and 40.

For example, each target 34a, 36a, 38a and 40a of the various targetassemblies has a height of 16 inches and a width of 11 inches. Theshutter apertures 80 and 82 have a height of 151/2 inches and a width of101/2 inches.

A source of gas (e.g., argon) is connected to the apparatus via a pipe86 extending through the base plate 14.

The various apparatus portions above-referred to, e.g., the pallets 46,the carrousel 54, the shutter 66, etc., are preferably made of stainlesssteel.

As previously indicated, apparatus for sputter depositing materials ontoworkpieces are well known, and structural and processing details, e.g.,the electrical power supplies, the various control circuits, the targetassemblies, and the like used with the apparatus 10 are known and amatter of choice to workers in these arts.

INTERNAL HIGH VACUUM PUMP

Of particular importance is the disposition of a high vacuum pump 18directly within and centrally of the the apparatus 10 itself, i.e.,along the longitudinal axis of the work treating chamber of theapparatus. In general, rotary vacuum treating apparatus of the typeherein described are known (see, e.g., U. S. Pat. No. 3,400,066, issuedto Caswell, et al. on Sept. 3, 1968, the disclosure thereof beingincorporated herein by reference), an advantage of such apparatus beingthat relatively large numbers of workpieces can be processed in eachbatch processing cycle of operation. One limitation on the productionrate of such known apparatus, however, is the time it takes to pump downthe apparatus.

In apparatus of the type described herein, the disposition of the pump18 directly within the apparatus greatly reduces the pump down time.This is because the gas conductance, i.e., the rate of flow of gases tothe gas collecting or high vacuum pump, is not limited by thecross-sectional area, length, or shape of ducts leading from theapparatus to the pump. In effect, the cross-sectional area with respectto the conductance of gases to the pump 18 in apparatus according to theinstant invention is in the order of the peripheral area of thecentrally located pump itself, i.e., the area of a cylinder encirclingthe pump. Also, being centrally disposed and within the apparatusitself, the path length for gases to the pump is at a minimum.

In the instant embodiment, the pump 18 (FIG. 1) is a known type ofcryogenic pump comprising a plurality of vertical tubes 90 mountedbetween a pair of horizontal annular tubes 92. Liquid nitrogen is pumpedinto the lower tube 92 via an entrance pipe 94, rises through thevertical tubes 90 to the upper tube 92, and is vented through a verticaltube 95 to an exit pipe 96. This type of pump collects gases whichfreeze at a temperature higher than that of liquid nitrogen, e.g.,oxygen and water vapor, such gases freezing and collecting on the coldouter surfaces of the pump elements.

When exposed to the surrounding apparatus, i.e., when the pump enclosure20 is in its raised position (FIG. 1), substantially the entire surfacearea of the pump is directly exposed to the surrounding apparatus andthe apparatus gases can flow along basically unimpeded, direct paths tothe pump. Thus, the effective cross-sectional area for gas flow to thepump is quite high, and is significantly greater than that ofconventional ducts normally used to connect high vacuum pumps tochambers being evacuated. Accordingly, in comparison with conventionalvacuum apparatus employing conventional high vacuum pumps disclosedexteriorly of the chamber to be evacuated, the instant arrangementprovides a significantly higher rate of gas collection or pumping speed.

For example, it is calculated that the pumping speed of apparatus inwhich the high vacuum pump is disposed within the apparatus, as hereindisclosed, is about twice as fast as when the identical high vacuum pumpis disposed outside the apparatus and coupled thereto by a 6 inchdiameter duct having a length of 10 inches.

Other types of known high vacuum pumps can be used, e.g., magnetic ionor differential ion-type pumps. Similarly, a cryogenic pump comprisingpanels chilled by a peltier process rather than by cryogenic fluids canbe used.

OPERATION

With the pump 18 enclosure 20 in its downward, sealed relation aroundthe pump 18, the main enclosure 12 is lifted and a number of waferloaded pallets 46 are mounted on the carrousel 54. The main enclosure 12is then lowered to seal the apparatus 10, and pump down of the apparatusis begun by the roughing pump through the duct 16. When the pressurewithin the apparatus reaches about 100 torr, the tandemly arrangedturbomolecular and foreline pumps are used to further evacuate theapparatus through the duct 17 to a pressure of about 5 × 10⁻ ⁵ torr.Thereafter, the pump enclosure 20 is raised within the apparatus 10 toexpose the pump 18 to the surrounding portions of the apparatus, thepump 18 then cooperating with the turbomolecular pump, which remains inoperation throughout all the following operations of the apparatus, toreduce the pressure to about 5 × 10⁻ ⁷ torr.

As previously described, the presence of the pump 18 directly within theapparatus 10 significantly increases the rate at which gases arecollected during the high vacuum pump down process.

When titanium or other reactive materials, such as tantalum, is one ofthe materials sputter deposited in the inventive apparatus, a furtherpumping sequence is preferably done prior to the start of the workpiecedeposition process. Thus, the pump enclosure 20 is again lowered intosealing relation about the pump 10, and argon gas is admitted into theapparatus at a flow rate sufficient to maintain the pressure within theapparatus at a pressure of between about 5 × 10⁻ ³ to 10⁻ ² torr. Atthis time, as shown in FIG. 2, each shutter aperture 80 and 82 isdisposed about 145° (center to center) counterclockwise from the target34a and 38a, respectively, with which it will shortly cooperate, theworkpiece pallets 46 thus being totally shielded from the targets 34aand 38a by the shutter 66. A voltage differential of about 2000 volts ata frequency of 13.5 mHz is then applied between the shutter 66 and thetwo targets 34a and 38a to initiate sputtering of these targets. Theshutter 66 is then rotated about 110° in a clockwise direction to exposesubstantially the entire outer surface of the shutter 66 (but not thepallets 46) to the targets 34a and 38a. Thus a large area layer oftitanium is deposited onto the shutter.

The titanium or other reactive material layer is provided on the shutter66 for two reasons. One, as is conventional, a preliminary sputtering ofthe targets is done to clean the targets of various surfacecontaminants. Second, by providing the large area titanium layer, aquite effective getter for reactive gases, e.g., oxygen, nitrogen, andwater vapor, is provided.

To continue the further pumping sequence, the flow of argon is thendiscontinued, the apparatus 10 is again pumped down to about 5 × 10⁻ ⁵torr by the turbomolecular pump, the high vacuum pump 18 is againexposed, and the apparatus is then further pumped down to about 2 × 10⁻⁷ torr by the pump 18 and the turbomolecular pump. The presence of thetitanium gettering layer significantly reduces the time required to pumpdown the system from 5 × 10⁻ ⁷ torr to 2 × 10⁻ ⁷ torr. That is, since apreliminary sputtering of the titanium targets must be done to clean thetargets, such preliminary sputtering is utilized to provide a more rapidand efficient pump down of the system.

Having now achieved a quite low pressure, for the dual purpose ofdemonstrating that the system is substantially leak-tight and providingan environment substantially free of contaminating gases, the depositionprocess is started. Thus, the pump 18 is again isolated from thesurrounding apparatus by the enclosure 20, argon is readmitted into theapparatus at a rate to maintain therein a constant pressure of betweenabout 5 × 10⁻ ³ to 10⁻ ² torr (the pump 18 being isolated to prevent itsbeing unnecessarily loaded with argon), and a sputtering voltage isapplied between the pallets 46 and the targets 34a and 38a to againcause sputtering of these targets.

At this time, the shutter apertures 80 and 82 are disposed about 35° ina counterclockwise direction from their associated targets 34a and 38a,respectively (see FIG. 5, which shows, schematically, only a portion ofthe apparatus), the workpiece pallets 46 still being shielded from thetargets 34a and 38a by the shutter 66. In the portion of the depositionprocess about to be described, the two oppositely disposed targets 34aand 38a are simultaneously sputtered, and pallets 46 on opposite sidesof the carrousel 54 are simultaneously coated with titanium, one-halfthe pallets 46 being exposed to and coated by the target 34a, and theremaining pallets 46 being coated by the target 38a. The purpose of thissimultaneous sputtering is to decrease the time required to process allthe workpieces. The cooperation among the carrousel 54, the shutter 66,and the targets 34a and 38a is identical at each side of the apparatus,hence the following description of the deposition from the target 34a isalso applicable to the simultaneous deposition from the target 36a.

At the start of the deposition process, both the shutter 66 and thecarrousel 54 are rotated in synchronism, i.e., at the same angular rate,to advance both the aperture 80 and those particular pallets 46 at thistime directly behind, and thus exposed by the aperture 80, across theface of the target 34a. That is, at the start of the deposition cycle, aparticular group (FIG. 5) of three pallets (designated 46a, 46b, and46c) is in direct alignment with the shutter aperture 80, the edges 100and 102 of the aperture 80 being in radial alignment with the outsideedges 104 and 106 of the outside pallets 46a and 46c. This particularalignment is maintained as both the shutter 66 and the carrousel 54 areinitially rotated to advance the aperture 80 across the face of thetarget 34a. During such advancement, material from the target 34a issputter deposited onto the thus exposed pallets 46a-46c.

When the aperture 80 arrives in full alignment with the target 34a,i.e., on centers therewith (FIG. 6), further movement of the shutter isstopped while rotation of the carrousel is continued at a constant rate.At this instant, those three pallets 46a through 46c then exposedthrough the aperture 80 to the target 34a begin passing beyond theleading edge 100 of the aperture 80 and behind the shutter 66.Similarly, successive pallets 46d, 46e, etc., on the rotating carrousel54 begin reaching the trailing edge 102 of the aperture and are exposedto the target 34a as they pass the now stationary aperture 80. Passingthe pallets 46 at a continuous rate past the target 34a, it is found, isa simple and effective means for uniformly depositing material acrossthe face of each workpiece.

The purpose of the described relative movements of the shutter andcarrousel is to make full use of the pallet mounting capacity of thecarrousel while insuring that all the pallets are uniformly exposed tothe target. For example, if the shutter aperture 80 were fixedlypositioned in alignment with the target 34a, any pallets in alignmentwith the aperture at the start of the sputtering process would not beidentically exposed to the target as those pallets which move, at acontinuous rate, from a position behind the shutter, into alignment withthe aperture, and then again behind the shutter. That is, using acontinuous rate of rotation of the carrousel 54, those pallets which areinitially exposed to the target at the start of the sputtering process,and thus already at least partly advanced across the face of the target,receive less than the normal amount of exposure to the target as thesepallets are rotated further past the target and behind the shutter.

To avoid this under exposure of these initially exposed pallets, onesolution, with an immobile shutter, is to provide a gap in the palletarray so that no pallets are in alignment with the shutter aperture atthe start of the sputtering cycle, all the pallets thus experiencing thesame exposure to the target. A disadvantage of this, however, is thatthe production capacity of the apparatus is reduced in accordance withthe size of the pallet gap.

An alternate solution, with an immobile shutter aperture and palletsdisposed in alignment therewith at the start of the sputtering cycle, isto re-expose those underexposed pallets to the target for a timeadequate to compensate for the initial underexposure thereof. Adifficulty with this, however, is that the separate exposure of theworkpieces provides separate and discrete layers of the depositedmaterial; there being, for certain materials, a distinct interfacebetween the separately deposited layers. In some instances, dependingupon the particular materials involved and how the workpieces are to beused, this is objectionable. With the relative shutter-carrouselmovement sequence described above, however, uniform exposure of thevarious workpieces is obtained without the need for separate,compensating re-exposures.

Towards the end of the titanium deposition cycle, the shutter 66 isagain moved in synchronism with the carrousel 54. Thus, as shown in FIG.7, when the trailing edge 106' of the last pallet 46n to be exposed tothe target 34a is rotated into radial alignment with the trailing edge102 of the aperture 80, the shutter 66 is again rotated in synchronismwith the carrousel 54 such that the alignment of the two edges 102 and106' is maintained as the last pallet 46n (the "nth" pallet, in thisembodiment, being the 13th pallet) is moved across the face of thetarget 34a. The result of this is that the last pallet 46n is exposed tothe target 34a exactly as all the other pallets were exposed, while thepallet immediately following the last pallet, i.e., the first pallet46a' of the other half of the pallets which had been exposed to theother target 38a, is shielded from the target 34a and thus not subjectto having a separate layer of titanium deposited thereon.

After the titanium deposition cycle, the platinum targets 36a and 40aare pre-sputtered to clean them of surface contaminents, and theabove-described deposition process, including the synchronous movementsof the shutter 66 and the carrousel 54, is performed to deposit platinumon the workpieces. This completes the workpiece treating process.

As above-described, the cryogenic pump 18 collects and stores gases onthe outer surfaces thereof. Periodically, during cleaning of theapparatus, the pump 18 is heated, as by passing hot nitrogen through thepump tubes 90 and 92, and the frozen gases are "boiled" off the pump.

While the invention has been described in connection with a sputteringapparatus, the invention has utility in other types of vacuum treatingapparatus, e.g., filament evaporation and electron beam evaporationcoating machines.

What is claimed is:
 1. In a method of depositing a material onto a workassembly in which different portions of the work assembly aresuccessively exposed to a source of the material by moving the workassembly past a shutter opening in alignment with said source, theimprovement comprising:initiating emission of said material from saidsource in directions towards said work assembly while said shutteropening is disposed out of of alignment with said source, said shutterthus shielding said work assembly from said source; synchronouslyadvancing said opening, and a particular portion of said work assemblyin alignment with said opening, into the path of material from saidsource until said opening is in fully aligned relation with said source;while said opening is maintained in said aligned relation, continuingmovement of said work assembly past said opening to expose successiveportions of said work assembly to said source; and when the trailingedge of the last portion of said work assembly to be exposed to saidsource reaches the near edge of said opening, again synchronouslyadvancing said shutter and said work assembly until said work assemblyis fully shielded from said source by said shutter.